First thing you need to do is go to your WordPress admin area and click on Plugins » Add New. Then search any plugin and click on install button. But from here you can only install free plugins that are available in WordPress plugin directory. Click on image left from more info.

For this first you need to extract plugin zip file. Now Open the FTP client on your computer and connect. Once connected, you need to access the path /wp-content/plugins/ Next, upload the folder you extracted from the zip file to the /wp-content/plugins/ folder on your web server. Then you can activate your plugin from installed plugins list in WordPress dashboard.